Vaneless diffuser



Dec. 6, 1966 SHAO 1.. $00 3,

VANELESS DIFFUSER Filed Oct. 8, 1965 5 Sheets-Sheet 1 INVENTOR.

SHAO L. 500

ATTORNEYS Dec. 6, 1966 SHAO L. $00 3,289,921

VANELESS DIFFUSER Filed Oct. 8, 1965 5 Sheets$heet 5 2 o G I 5 -0.080.06 o.o4 .o.oz 0 0-0! INVENTOR 5 HA 0 L- 5 00 United States Patent3,289,921 VANELESS DIFFUSER Shao L. $00, Urbana, 111., assignor toCaterpillar Tractor (10., Peoria, Ill., a corporation of CaliforniaFiled Oct. 8, 1965, Ser. No. 494,225 3 Claims. (Ql. 230127) Thisapplication is a continuation-in-part of my copending application forVaneless Diffuser, Serial No. 328,03 8, filed December 4, 1963, nowabandoned.

The present invention relates to the diffuser section of a turbomachine,such as a compressor or turbine and particularly to an improved contourof diffuser walls for increasing the etficiency of a compressor or thelike.

In the past, vaneless diifusers have been made with parallel wall-s,diverging walls and constant flow area configurations, without regard tospecific control of boundary layer separation and attendant wallfriction losses.

It is the object of the present invention to provide a vaneless diffuserspecifically contoured to obtain maximum efliciency with a minimum ofloss due to friction and heat.

The invention is disclosed herein in combination with a radial flowcompressor but is equally useful with axial and mixed flow compressorsor turbines for fluid pumpmg.

The manner in which the invention is carried into practice will best beunderstood from the following specification wherein reference is made tothe accompanying drawings.

In the drawings:

FIG. 1 is a central sectional view through a typical radial compressorand diffuser therefor showing a diffuser embodying the presentinvention;

FIG. 2 is a fragmentary view in section showing a modified diffuserprofile;

FIGS. 3 to 6, inclusive, are schematic views illustrating typical flowpatterns of fluid through a confined space such as a diffuser; and

FIG. 7 is an enlarged view in cross section of the diffuser profile ofFIG. 1 showing flow patterns at different positions therein.

FIG. 8 is a graph of boundary [layer parameter G as a function ofboundary layer parameter I,.

The compressor shown in FIG. 1 includes a housing 10, a shaft 12 drivenby means not shown to rotate an impeller 14 thereon, the impeller havingblades 15. Fluid to be compressed enters the housing axially of theshaft from the left as shown in FIG. 1 and is compressed by the impellerand directed radially outwardly at its periphery through an annularvaneless diffuser consisting of a pair of axially spaced annular wallsshown at 16 in FIG. 1 into a collector, 17. In FIG. 1 both walls of thediffuser are contoured according to the teaching of the presentinvention. A modification is shown in FIG. 2 where a similar diffuser isshown as having one flat wall and the other wall contoured to obtain asimilar result.

The present invention is predicated upon producing a diffuser profile byapplying known formulae to produce a condition of optimum flowthroughout the greater part of its length. The efliciency of fluid flowthrough a confined space is determined largely by boundary layerconditions for the action of the fluid adjacent the walls of the space.Some boundary layer conditions are illustrated in FIGS. 3m 6, inclusive,which show velocity profiles of a fluid flowing within passage walls. Asshown in FIGS. 3, 4 and 5, the flow at the main or central portion ofthe fluid stream is relatively uniform, while the boundary layer or flowadjacent the passage walls varies considerably under differentconditions of velocity, pressure, temperature, density and otherfactors.

Under certain conditions, there exists an unseparated boundary layerhaving a velocity gradient adjacent to the walls of less than asillustrated in FIG. 4. This produces high friction and conversion ofkinetic energy to heat. FIG. 5 represents a condition producing aseparated boundary layer where there is a greater than 90 gradient and areverse curve in the velocity profile. This reverse curve is shown bythe enlargement in FIG. 6 illustrating the reversal of flow in thevelocity profile and representing an even greater loss in elliciency andflow range than the condition illustrated in FIG. 4.

The optimum conditions are represented in FIG. 3 which illustratesimminent boundary layer separation with a gradient adjacent the wall ofapproximately 90.

The present invention resides in the provision of a diffuser whichproduces the efiicient velocity profile of the type illustrated in FIG.3 throughout the greater part of its length. This is accomplishedthrough a convergingdiverging-converging diffuser wall contour asrepresented by the areas 18, 19 and 20 of FIG. 7.

A first area represented by the distance 18 in FIG. 7 has an entryportion of converging cross section which is provided to produceacceleration up to the point of a throat, indicated at 21, whichacceleration reduces turbulence created by the vanes 15 of thecompressor. A second area 19 has a diverging cross section and causesdiffusion up to the point 22 where imminent boundary layer separation isreached. In communication with and extending downstream from this pointand to the point of discharge as represented by a third area 20, thewalls of the diffuser are contoured to maintain imminent boundary layerseparation as represented by the three typical velocity profiles 2'5, 26and 27 illustrated in this area.

The following presents one application of known formulae to illustratethe detailed computation procedure followed in producing the diffuserprofile for optimum flow. Other procedures could be followed to producesimilar profiles.

A. Nomenclature Pressure, 17

Density, p

Temperature, t

Dynamic viscosity, u

Kinematic viscosity, 1/

Boundary layer thickness, 5 Turbulent radial wall shear stress, 'rRadial Mach number, M,

B. Equations of fluid mechanics Beginning with the Navier-Stokesequations and the equation of conservation of mass in cylindrical coordinates, the assumptions of steady, compressible, axismymetric, boundarylayer flow results in the following momentum integral equation:

u dr

dimensional incompressible flow, the following is derived from Equation1:

a e] p u d1" 1 u Upon inserting the definition of I in Equation 2 alinearequation of first order in Y results:

1 du 2... r; u dr and K is a constant of integration.

C. D 'fiuser profile The solution of Equation 4 may be obtained byspecifying u, v, and G as functions of r and performing the requiredintegrations. Constant K may be obtained by specifying a certain valueof Y at r=r As an example, let u be given by:

where L is the diffuser wall spacing. Let v be given by:

(conservation of mass) T (free-vortex) G may be taken from FIG. 8. At aspecified r, trial and error yields 1 for specified L. Using Buris data,I, is restricted to the range:

(stagnation) (separation) 4 The difi'user profile illustrated in FIG. 7may be obtained by specifying the wall spacing L in region 18 inaccordance with the following equations:

where, from NACA Technical Note 1426,

and

r =radius to throat 21 L =wall spacing at throat 21 The dimensions r andL are given by the required design conditions of the diffuser, such asdesign mass flow and pressure ratio.

The difiuser profile in region 19 may also be proportioned in accordancewith NACA Technical Note1426, as follows:

i tan \V 1'1- v2 Sir/2A1 where #1 may vary between 4 and 10 degrees.

Calculation of L in region 19 is terminated when I Equation 4, reachesthe imminent separation value of approximately .055. The Wall spacing inregion 20 is computed by keeping l =.055=constan-t so as to maintainimminent boundary layer separation to the diffuser discharge.

The above example is presented as an illustration only. Other procedurescould be used to obtain the non-separating vaneless diffuser shape showngenerally by FIG. 7. The diffuser flow could be unsteady ornon-symmetric. The boundary layers could be laminar as well asturbulent. The fiow could be axial, as the discharge from an axialturbine, or at any intermediate angle with respect to the shaft 12.

References:

(1) H. Schlichting, Boundary Layer Theory, Mc- Graw-Hill, 1955.

(2) S. P-ai, Viscous Flow Theory, Van Nostrand, 1957.

(3) NACA Technical Note 1426, 1947.

I claim:

1. A vaneless diffuser for a radial fiow turbomachine, said ditfusercomprising a pair of axially spaced annular walls about the periphery ofan impeller and defining a vaneless diffuser fioW passage thereabout,the radially inner portion of the axially spaced walls defining an entryportion for pumped fluid to said diff-user passage, the radially outerportion defining a discharge point wherein said pumped fluid enters asurrounding collector in an unrestricted manner, said Walls of saidentry portion defining a first converging area to a throat foraccelerating fluid and reducing turbulence in the fluid flow, saidaxially spaced walls defining a diverging second portion of said flowpassage to define an area of increasing cross-section to a point in thedownstream flow where boundary layer separation is imminent from saidaxially spaced Walls in said second portion of said fiow passage, saidaxially spaced walls defining a third portion of said flow passage incommunication With said second portion and extending downstream of saidsecond port-ion of said flow passage to the point of dischargetherefrom, said axially spaced walls of said third portion defining ameans for maintaining the condition of im- 5 6 minent boundary layerseparation from said walls through- References Cited by the Examiner outthe third portion of said flow passage. FOREIGN PATENTS 2. A vanelessdiffuser for a radial flow turbomachine according to claim 1 whereinsaid axially spaced annular walls are arranged symmetrically about aplane midway 5 between said Walls.

3. A vaneless diffuser for a radial flow turbornachine according toclaim 1 wherein one of said axially spaced annular walls issubstantially flat. HENRY F. RADUAZO, Examiner.

855,124 2/ 1940 France.

26,368 1907 Great Britain. 336,840 10/ 1930 Great Britain.

DONLEY J. STOCKING, Primary Examiner.

1. A VANELESS DIFFUSER FOR A RADIAL FLOW TURBOMACHINE, SAID DIFFUSERCOMPRISING A PAIR OF AXIALLY SPACED ANNULAR WALLS ABOUT THE PERIPHERY OFAN IMPELLER AND DEFINING A VANELESS DIFFUSER FLOW PASSAGE THEREABOUT,THE RADIALLY INNER PORTION OF THE AXIALLY SPACED WALLS DEFINING AN ENTRYPORTION FOR PUMPED FLUID TO SAID DIFFUSER PASSAGE, THE RADIALLY OUTERPORTION DEFINING A DISCHARGE POINT WHEREIN SAID PUMPED FLUID ENTERS ASURROUNDING COLLECTOR IN AN UNRESTRICTED MANNER, SAID WALLS OF SAIDENTRY PORTION DEFINING A FIRST CONVERGING AREA TO A THROAT FORACCELERATING FLUID AND REDUCING TURBULENCE IN THE FLUID FLOW, SAIDAXIALLY SPACED WALLS DEFINING A DIVERGING SECOND PORTION OF SAID FLOWPASSAGE TO DEFINE AN AREA OF INCREASING CROSS-SECTION TO A POINT IN THEDOWNSTREAM FLOW WHERE BOUNDARY LAYER IN SAID SECOND PORTION OF SAID FLOWPASSAGE, SPACED WALLS IN SAID SECOND PORTION OF SAID FLOW PASSAGE, SAIDAXIALLY SPACED WALLS DEFINING A THIRD PORTION OF SAID FLOW PASSAGE INCOMMUNICATION WITH SAID SECOND PORTION AND EXTENDING DOWNSTREAM OF SAIDSECOND PORTION OF SAID FLOW PASSAGE TO THE POINT OF DISCHARGE THEREFROM,SAID AXIALLY SPACED WALLS OF SAID THIRD PORTION DEFINING A MEANS FORMAINTAINING THE CONDITION OF IMMINENT BOUNDARY LAYER SEPARATION FROMSAID WALLS THROUGHOUT THE THIRD PORTION OF SAID FLOW PASSAGE.